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The
Secret Life of the Stirling Engine

Happy
New Year! I can
report that I am very excited about what this year will
bring. We begin, however, with a glance into the past
with this special Video Edition of Making
Sense of Science. As the product of a bygone era
overtaken by history and technology, the Stirling Engine has
spent most of its existence in obscurity in the science
classroom as an instructive curiosity. In recent times
however, it has seen something of a revival as people search
desperately for solutions to the conflict between the
expanding energy needs of the world's population and the
long-term consequences of energy use.

Though it has been around
for nearly 200 years, the Stirling Engine has never made much
of an economic impact on the world. There is a reason
for this, which a scientific approach quickly reveals.
To view the video, click on this link:

There is something about the way that a Stirling Engine
converts fire into mechanical motion that fascinates both
amateur tinkerers and professional engineers alike (who really
ought to know better). I too confess to having been
bitten by the Stirling bug in the past, and have built a
number of experimental engines while contemplating their
improbable commercial application.

As if by magic the Stirling Engine seems to violate common
sense and the 2nd Law of Thermodynamics by apparently
reversing the natural flow of energy. That flow is
always from concentrated forms (like mechanical motion) ever
towards diffuse forms (like heat), and never the other way
around. Actually, the Stirling Engine and other devices
(including refrigeration) fully comply with natural law by
speeding up the diffusion of heat energy while reversing the
flow of just a small proportion of it.

While most proponents of the Stirling Engine focus their
attention on thermodynamic efficiency (the amount of
mechanical energy recovered compared to the amount of heat
energy supplied), the engine suffers from problems of much
greater significance than efficiency.

I know I will be bombarded with emails from Stirling Engine
advocates pointing out "such and such" advantage or "this and
that" recent development. But none of that changes the
fact that for their size, weight, displacement, cost per
capacity, service life, or by any other measure, Stirling
Engines simply underperform conventional engine and turbine
technology. Efficiency alone just doesn't matter as
much as overall Return On Investment.

Of course the model Stirling Engine used in the video is
not optimised for performance, but for simplicity. By
their nature, however, all Stirling Engines operate at
relatively low internal pressures and compression ratios, and
this is the crux of the matter. Low pressure means low
specific work and low power output.

In contrast, Rudolf Diesel, a keen student of Sadi Carnot
(the father of thermodynamics theory), understood this point
very well and made full use of it in the design of his
engine. Diesel's engine has excellent characteristic
Return On Investment as evidenced by its widespread
utilization in industry.

By way of comparison, the model Stirling engine shown in
the video weighs about 330g without the base and produces
around 0.012W of power. A 330g model aircraft engine can
be expected to produce nearly 1000W of power. This means
one model internal combustion engine can replace 80,000 model
Stirling engines of the same weight. Surely, some
improvements can be made, you say. I quite agree.
However, an 80,000-fold improvement is a tall order, certainly
costing millions in R&D.

Undeterred, advocates of the Stirling Engine have spent
many millions of dollars over the decades on improved Stirling
Engine designs. A design called a Free Piston Stirling
Engine (because it has no crankshaft) is contained in a
sealed, pressurized vessel and generates electricity when
heated by sunlight concentrated by an array of motorized
mirrors. Will it work? I am sure that it
will. I am equally sure that it will be
ridiculously expensive for the amount of
electricity ultimately produced.